CH. XXIV.] 



THE LUNGS 



347 



relatively more abundant than in the larger ones, and form a 

 distinct circular coat. 



The Lungs and Pleurae. The lungs occupy the greater portion of 

 the thorax. They are of a spongy elastic texture, and are composed 



FIG. 321. Transverse section of the chest. 



of numerous minute air-sacs, and o'n section every here and there the 

 air-tubes may be seen cut across. Any fragment of lung (unless 

 from a child that has never breathed, or in cases of disease in which 

 the lung is consolidated) floats in water ; no other tissue does this. 



Each lung is enveloped by a serous membrane the pleura, one 

 layer of which adheres closely to 

 its surface, and provides it with its 

 smooth and slippery covering, while 

 the other adheres to the inner sur- 

 face of the chest-wall. The con- 

 tinuity of the two layers, which 

 form a closed sac, as in the case of 

 other serous membranes, will be 

 best understood by reference to fig. 

 321. The appearance of a space, 

 however, between the pleura which 

 covers the lung (visceral layer) and 

 that which lines the inner surface 

 of the chest (parietal layer) is in- 

 serted in the drawing only for the 

 sake of distinctness. It does not 

 really exist. The layers are, in health, everywhere in contact one 

 with the other; and between them is only just so much fluid as will 

 ensure the lungs gliding easily, in their expansion and contraction, 

 on the inner surface of the parietal layer, which lines the chest-wall. 



FIG. 322. Ciliated epithelium of the human 

 trachea, a, Layer of longitudinally arranged 

 elastic fibres; b, basement membrane; 

 c, deepest cells circular in form ; d, inter- 

 mediate elongated cells ; e, outermost layer 

 of cells fully developed and bearing cilia. 

 x 350. (Kolliker.) 



